Mobile communication systems have been developed with the aim to facilitate communication everywhere, with everyone and at any time. In recent years, mobile communication systems, and particularly cellular communication systems, experienced a huge increase, both in the number of users and in the quality and demands of services offered.
Commonly used and widely spread mobile communication systems such as the pan-European Global System for Mobile Communication (GSM) are cellular systems. A cellular system or network is characterized in that it is organized on a cell basis, wherein each cell comprises a base station whose radio coverage area defines the geographical spreading of this cell. Since only a limited frequency band is available for an entire mobile communication network and each communication channel requires a certain bandwidth, it is essential to exploit the available frequency band as efficient as possible such that as many users as possible can be serviced in the network. Therefore, in a cellular network, the available frequencies are usually reused on a cell basis. This means that the same set of frequencies, i.e. the same frequency band, which is used in one cell are also used in another cell of the same system in order to increase the user capacity of the system. However, in this regard, there exists a drawback in that interferences between the communications of users in different cells may occur, when the same frequencies are used. Such interferences are desired to be avoided since the communication quality is deteriorated due to them. Thus, the same frequencies are to be reused only in cells which are spaced at a minimum distance from each other. However, the larger the spatial frequency reuse is and, thus, the lower potential interferences are, the fewer users can be serviced in the system, i.e. the lower the capacity of the system is. That is the spatial frequency reuse is desired to be as small as possible, in particular in view of an increasing number of users.
In order to cope with the increasing requirements mentioned above in terms of number of users and demands of services, which are posed on mobile communications, mobile systems and networks of the third generation (3G) and even the fourth generation (4G) are under development and partly already in operation, e.g. the General Packet Radio Service (GPRS) and the Universal Mobile Telecommunication System (UMTS). Some of these systems are based on code division multiple access (CDMA) techniques.
The current working assumption for a 4G cellular system in a high frequency bandwidth requirement amounts to 1 Gbps (Gigabits per second) in maximum data rate. To achieve reasonable multi-operator scenarios in view of suchlike requirements and with limited total bandwidth availability, the frequency reuse factor in the network must be low. Further, for a continuous coverage of the whole cell by its base station, pilot and broadcast channels must be receivable over the whole cell area, which may also result in overlaps with neighboring cells. However, such overlaps are adverse with respect to the aim of a smaller frequency reuse factor, since overlaps between cells using the same frequency bands would again result in deteriorating interferences.
Generally, an overlapping can be avoided or, at least, reduced by accordingly affecting the transmission powers of cells using the same frequency band. A method for channel allocation utilizing power restrictions is presented in U.S. Pat. No. 6,259,685. In this method the time-slotted transmissions of synchronized base stations are arranged in such a way that transmissions utilizing maximum power P do not occur at the same time t in cells sharing the same frequency band.
The principle of a time-slotted transmission power scheme according to the prior art solution cited above is illustrated in FIG. 1. The figure shows the power restrictions of the base station for a situation of three neighboring cells, with P denoting the transmission power of the base station of the respective cell and t denoting the time. In a normal situation, the single timeslots are allocated to different terminals at different geographical locations.
This prior art method results in a kind of “breathing” in the cell coverage areas, which would in this case be the desired outcome producing the spatial overlap in the border zone between two cells, enabling camping of a mobile station on either cell.
In U.S. patent application Ser. No. 10/937,296, which is also owned by the present applicant and which was not yet published at the date of filing of the present application, there is proposed a time-based frequency reuse scheme. In the thus proposed solution, there are defined one or more time slots in the time frame structures of neighboring base stations, in which time slots cell management information relating to the respective cell is to be transmitted. Thereby, the defined time slots of neighboring base stations are shifted against each other on a time basis, and thus pilot and broadcast channel transmissions of neighboring base stations can be guaranteed to occur at different times, as can be gathered from FIG. 2.
Among others, there are shortcomings of the prior art solutions according to FIGS. 1 and 2 in that the arrangement and properties of time slot transmissions are static after their establishment. Thus, the known solutions do not provide for a flexibility in view of changing conditions, and hence are not sufficiently efficient considering today's demands.
Thus, a solution to the above problems and drawbacks is desirable for a cellular communication network, in which frequency reuse possibilities are limited.